Formed composite parts are commonly used in applications, such as aircraft and vehicles, where lightweight and high strength are desired. Fabricating composite parts, for example a ply-by-ply airplane wing stringer, requires the use of a lay-up tool (also known as a base tool, forming tool or mandrel) on which layers of composite materials or pre-cure lamina are hand formed or laid one ply at a time. These applications can utilize complex contoured finished parts or channels that must be formed and then cured. Historically, complex contoured composite structures have entailed extensive hand labor to form them prior to curing. Typically, the pre-impregnated composite fiber plies (“pre-pregs”) such as epoxy impregnated carbon fiber laminates are laid by hand or by a machine over a tool or mandrel. Once the desired numbers of layers are placed on the tool, the composite laminate is bagged and cured in a curing apparatus, such as an autoclave. Unfortunately, laying-up of a composite charge over a complex shaped forming tool creates several challenges and must be done manually by hand. It is time consuming and expensive to perform ply and ply lay-up.
One forming method, known as drape forming, uses vacuum bagging. Drape forming has been used successfully to form composite parts where the parts being formed are formed over tools that only have male radius. This method involves heating a flat laminate pre-preg composite blank or charge and forcing it around a forming tool with the use of a vacuum bag. However, this method has met with limited success on tools with female radius or more complex shapes.
Vacuum bag drape forming of such parts can often result in wrinkling and/or pinching of the plies. Wrinkles occur because some laminate plies are in compression when bent or urged over the forming tool, and buckle when there is no constraint on the bending portion to prevent out-of-plane-buckling. Similarly, on long flange parts, slip resistance between the plies during bending becomes too great, and inner plies buckle. Buckling or wrinkling of the plies also occurs over tools that are curved or contoured, or have joggles along their length. Even slight contours of a radius on the order of thousands of inches is enough to initiate wrinkles.
In particular, when using a Z-shaped tool, the male and female radii create challenges in the hot drape process as the charge may only form over the male radius. As the composite pre-preg charge is bent over the forming tool, if the length of the flange is too long or slip resistance between the plies is too great, out-of-plane-buckling of the laminate will occur. This results in the use of a time consuming two-step process where a special tool is used to form the male radius and then separately to form the female radius, typically using hand lay-up techniques.
Although there has been limited success with use of release films applied to the base tool, because these films have inherently have low tack properties they do not provide a stable and secure surface for the lay-up of the plies of composite pre-preg material. This can result in slower than desired laydown rates and an increase in scrapped parts.
Accordingly, there is a need for improved hot drape systems and methods of laying-up composite plies over a complex and contoured forming tool that avoids or minimizes hand lay-up processes and wrinkling that is experienced with prior known methods.